The present invention relates generally to tundishes used in the continuous casting of molten alloys, such as molten steel, and more particularly to a tundish constructed to control or direct the escape from the tundish of an undissolved, molten alloying ingredient denser than the molten alloy as a whole.
The following discussion is in the context of molten steel containing undissolved, molten lead and/or bismuth as the denser alloying ingredient. However, that particular steel is merely an example of one type of molten alloy with which the present invention is intended to be employed; the present invention may also be employed with other molten alloys having similar characteristics, e.g. a molten copper-based alloy containing undissolved, molten lead.
In the continuous casting of molten steel, a stream of molten steel is poured from a ladle into an intermediate vessel known as a tundish having a bottom containing outlet openings through which molten steel flows into a continuous casting mold. Molten steel is conventionally introduced into the tundish at an entry location spaced from each of the outlet openings, and the molten steel normally flows along the bottom of the tundish downstream from the entry location to an outlet opening.
Certain steels, known as free-machining steels, contain lead and/or bismuth to improve the machinability of the steel. Typical contents for each are about 0.04-0.40 wt. % bismuth and 0.05-0.50 wt. % lead. Lead and/or bismuth my be added to the stream of molten steel entering the tundish.
Lead and bismuth have a relatively low solubility in molten steel, compared to other alloying ingredients added to molten steel, and lead and bismuth are denser than molten steel. Because of these properties, substantial amounts of undissolved lead and bismuth tend to accumulate at the bottom of the tundish. If these accumulations of undissolved lead and bismuth are allowed to flow out through the outlet openings in the bottom of the tundish, they will do so as relatively large globules, and this will be manifest in the solidified steel as large, localized concentrations of lead or bismuth, which is undesirable.
Various expedients have been employed to cope with the problems described in the preceding paragraph. Many of these expedients are described in Jackson, et al. U.S. Pat. No. 4,852,632, issued Aug. 1, 1989, and the disclosure thereof is incorporated herein by reference. One such expedient comprises interposing a refractory dam between the tundish entry location and the tundish outlet opening. This dam extends upwardly from the vessel bottom and prevents undissolved, molten alloying ingredient which settles on the tundish bottom from moving downstream past the darn. As used hereinafter, the term "undissolved, molten alloying ingredient" refers to undissolved molten lead or bismuth or other elements having like properties.
Expedients which prevent undissolved, molten alloying ingredient from entering a tundish outlet opening result in the accumulation of large amounts of undissolved alloying ingredient on the vessel bottom at a location spaced upstream from the tundish outlet opening, and that too is undesirable.
One proposal for preventing large accumulations of undissolved alloying ingredient on the vessel bottom comprises providing, at the bottom of the tundish, a sump located between the entry location and the tundish outlet opening. This sump has a floor which is lower than the tundish bottom surrounding the sump. The relatively dense, undissolved molten alloying ingredient collects in or about the sump as a result of the difference in density between the undissolved molten alloying ingredient and the molten steel. The sump floor is composed of a refractory material which is impermeable to molten steel but is permeable to the undissolved, molten alloying ingredient. A drain is provided in the metal shell of the tundish underlying the sump, and it is intended that the undissolved, molten alloying ingredient pass downwardly from the sump floor through the refractory material permeable to that alloying ingredient and then be removed through the drain in the underlying tundish steel shell. One embodiment of the sump described in the preceding part of this paragraph is disclosed in the aforementioned U.S. Pat. No. 4,852,632
Problems have arisen in the employment of the sump described in the preceding paragraph. More particularly, the passageways in the refractory, through which the undissolved, molten alloying ingredient was supposed to pass in the course of being removed from the sump, have become plugged with solidified or cooled, viscous alloying ingredient. This prevents removal of the undissolved alloying ingredient from the tundish bottom, causing the accumulations of undissolved, molten alloying ingredient on the tundish bottom to grow larger and larger. Some of this unremoved, undissolved, molten alloying ingredient may work its way underneath the refractory dam, behind which the undissolved, molten alloying ingredient is supposed to be contained; the undissolved, molten alloying ingredient can also work its way through cracks at the bottom of the refractory dam. When those things occur, the undissolved, molten alloying ingredient can flow downstream to the tundish outlet opening, which is undesirable. In addition, there may be other ways for unremoved, undissolved, molten alloying ingredient to find its way to the tundish outlet opening, all of this being undesirable.
In another embodiment, there is one dam upstream of the passageway, between the passageway and the tundish inlet location, and other dams downstream of the passageway, between the passageway and the tundish outlet openings. These dams wall off a portion of the tundish and define a tundish holding compartment. Molten steel flows over the tops of the dams and then flows to the tundish outlet openings. Undissolved, molten alloying ingredient accumulates in the holding compartment, and one or more passageways of the type described above are provided in the holding compartment to remove the accumulations. These passageways can be in the floor of a sump, or they can be merely in the bottom of the holding compartment, outside of a sump. In the case where a holding compartment is sump-less, the entire holding compartment is tantamount to one large sump.
The problem of plugged passageways, described above in connection with a passageway located in a sump floor, is also present when passageways are located in the bottom of a holding compartment, outside of a sump. When a passageway in the holding compartment becomes plugged, the accumulations of undissolved, molten alloying ingredient in that compartment become larger and larger, and that is undesirable.